The present invention relates to a noise canceler.
All electrical and electronic appliances use electricity represented by voltage and current. The flow of current through wire creates a magnetic field around the wire. Whenever a potential difference exists, electric and electromagnetic fields are produced in the surrounding space that vary over time. Also, when an AC voltage drop occurs, the supply voltage to a circuit is varied, which can cause the circuit to misoperate. Furthermore, these appliances generate electromagnetic waves which may impede the operation of other appliances or may cause their misoperation. Especially, noise produced during the starting of an automobile or the overhead passage of aircraft can result in the interruption of the normal operation of appliances.
FIG. 1 shows a conventional noise canceler that comprises a subtracter 1 having: a non-inverting input port (+) and an inverting input port (-); a subtracting signal IN.sub.2 entering via the inverting input port; a signal IN.sub.1 entering via the non-inverting input port; a signal OUT of which noise is canceled; and a capacitor C connected between the two input ports of subtracter 1.
A composite signal IN.sub.1 input via the non-inverting input port of subtracter 1 is composed of a low frequency signal and pilot signal IN.sub.2. The signal input via the inverting input port is a pilot canceling signal which has the same amplitude and phase as those of the pilot signal of the composite signal.
Subtracter 1 receives the composite signal IN.sub.2 via the non-inverting input port (+) and the pilot canceling signal IN.sub.2 via the inverting input port (-) so as to output the difference. The difference signal voltage is equal to the voltage applied between the two terminals of capacitor C connected between the non-inverting input port - and inverting input port - of subtracter 1.
Capacitor C blocks low frequency components and transmits high frequency components only. When the composite signal IN.sub.1 is applied to the non-inverting input port + of subtracter 1, the low frequency signal is output through the output port of subtracter 1. However, because the high frequency components present at the non-inverting input port (+) are coupled through the capacitor C to the inverting input port (-), any noise signal present in the composite signal IN.sub.1 is also present at the inverting port (-), and the input waveform at the non-inverting input port (+) of the subtracter 1 is the same as that at the inverting input port (-). This is because the input impedance of subtracter 1 is extremely high. Capacitor C, acting as a filter, is effectively an AC short so that, when a noise signal is contained in the composite signal IN.sub.1, the noise signal is simultaneously applied to both the non-inverting input port (+) and to inverting port (-) of subtracter 1. According to these operations, the noise signal is not passed to the output port of subtracter 1 and the subtracter output is the voltage difference between the direct current components at the non-inverting input port (+) and inverting input port (-).
However, in this configuration, the pilot signal and the noise signal of the composite signal IN.sub.1 pass through capacitor C and interfere with the pilot canceling signal applied to the inverting input port (-) of subtracter 1. Performance of the noise canceler is thus reduced.